McEwen, from Winnipeg, scored three in the first end but Pierce (2-2), from New Westminster, B.C., chipped away and pulled even in the fifth. McEwen scored single points in the last three ends for the victory to improve to 2-1.
In other early games, William Lyburn (1-3) of Winnipeg defeated Rob Rumfeldt (1-2) of Guelph, Ont., 4-3 while Toronto's Mark Kean (1-2) held off Peter de Cruz (1-3) of Switzerland 5-3.
Steve Laycock (2-2) of Saskatoon dumped Kevin Koe (2-2) of Calgary 8-4 and Brad Jacobs (3-1) of Sault Ste. Marie, Ont., topped Jean-Michel Menard (1-2) of Quebec City 7-5.
Brad Gushue of St. John's, N.L., and world champion Glenn Howard of Coldwater, Ont., are the only unbeaten rinks in the 18-team tournament at 3-0. Three more draws are scheduled for later Friday.
The tournament is the second men's Grand Slam event of the World Curling Tour season and has a purse of $100,000.